The four Zenith Stromberg carburettors

They were never the plan

Roger Bywater of AJ6 Engineering, who worked in Jaguar engine development, states that the V12 was designed from the outset to have electronic fuel injection, that the system was cancelled at a late stage, and that the four carburettor installation was improvised as a stopgap until Bosch D Jetronic could be introduced. Jaguar's public line in 1971 was more delicate: the engine had been designed to accept petrol injection although it was not intended to introduce the feature for the present.

Hassan and Mundy also conceded that the layout was forced partly by the non availability of a satisfactory British made downdraught carburettor. They admitted at the same time that the long inlet tracts were, in their words, frankly not very critical to the output of the engine.

D Jetronic reached the V12 on the XJ12 and the XJ-S late in 1975, after E-Type production had ended. No factory injected E-Type V12 has been established. Injected Series 3 cars are aftermarket conversions.

What is fitted

Four side draught Zenith Stromberg carburettors, two per bank, mounted outboard of the vee. They feed steeply angled inlet ports on the inner vee through plenums and inverted U shaped inlet manifolds. Fuel is supplied by twin SU AVF 106 electric pumps at 1.5lb per square inch. Throttle bore is 1.75 inches.

There is a disagreement about the designation. The April 1971 technical press called them Zenith 175CDSE. Every parts catalogue and every specialist the archive has consulted says 175CD2SE. The catalogues do split carburettor assemblies at engine numbers 7S4663 and 7S8175, which is consistent with a change during the run, but no source connects that split to a change of designation. The archive records both.

Needles and market differences

Metering needles differ by market and by engine number, and fitting the wrong one is a real cause of poor running on a car that has been apart. The parts catalogue lists three.

• B1BH, European specification, to engine number 7S8670
• B1CN, European specification, from engine number 7S8671
• B1CQ, United States and Canadian specification, from engine number 7S8179

Complete carburettor assemblies are catalogued in four engine number bands, split European against United States and Canadian, and split again between manual and automatic transmission cars. So a carburettor from another Series 3 is not necessarily the right carburettor for yours.

American cars carried emissions equipment including a vacuum retard on the distributor and an air injection pump, and from 1973 ran 7.8 to 1 compression against 9.0 to 1 elsewhere. American specialists routinely recommend removing the vacuum retard, deleting the air pump and converting to a mechanical advance distributor. That is a change from original specification and should be declared when a car is described.

The four things that go wrong

• Throttle spindle wear. Zenith's own technical material states that wear of the throttle spindle and its bearings will affect idle, eventually requiring replacement of the spindle or the whole carburettor. Note that the Series 3 catalogue lists three different throttle spindles for different positions, so they are not interchangeable between the four units.
• Diaphragms. They perish with age, and a set of new diaphragms is very often the difference between a car that runs badly and a car that runs properly. Zenith's factory position was that diaphragms and air valves were not to be changed in service and that sub assemblies should be replaced instead, which is a position the trade abandoned long ago.
• Balancing across four. This is the job that takes the time. Airflow has to be balanced across all four units, jet heights set, and the interconnecting rods balanced as well as the carburettors themselves. A specialist who does this work reports a target idle of 650 to 800rpm and typically achieves 725 to 750rpm.
• Hot fuel handling. Hot start failure caused by fuel vaporising in the carburettors and the fuel lines is a recognised Series 3 complaint. It is addressed with heat shielding and by rerouting fuel lines away from heat. American racing teams found the same problem, and reported that eliminating heat pickup from the long inlet route gained considerable power.

Cold starting

The Series 3 has a driver operated pull out choke control with a warning light on the dashboard, so cold enrichment is not automatic from the driver's point of view. The carburettors also carry an Automatic Enrichment Device, listed in the parts catalogue in several variants by engine number.

How the two relate is not established. The presence of both a driver's choke control and automatic enrichment devices suggests that the dashboard control operates the AEDs, but no source the archive has consulted states this, and the factory cold start procedure and fast idle figures for this engine have not been established. They are in the E165/2 workshop manual. A specialist quotes a cold fast idle of 1,200 to 1,300rpm as trade practice, which is not a factory figure.

In general terms an automatic enrichment device draws inlet air through a pipe from the exhaust manifold, and bimetallic springs progressively close the enrichment valve as that air heats, shutting the system down at operating temperature. Common faults are a blocked hot air pipe or filter, leaking rubber elbows at the manifold, plugged bleed holes in the valve disc, and flooding. That description is documented for Jaguar SU installations and has not been specifically confirmed for this Stromberg application.

Rebuild or replace

The trade view is consistent: these carburettors are worth rebuilding rather than replacing, and they respond well to it. Classic and Sports Car's buying guide puts it simply, that the Strombergs give trouble when old but are easily rebuilt with new diaphragms. Specialists working on American cars recommend re engineering the emissions compromised settings rather than throwing the carburettors away.

What none of them suggests is that four carburettors on a twelve cylinder engine can be set up quickly. The point a specialist makes about this job is the obvious one: the more cylinders an engine has, the more balancing and optimising is required.

Common questions